In the mouse circadian clock, a transcriptional feedback loop is at the center of the clockwork mechanism. CLOCK and BMAL1 are essential transcription factors that drive the expression of three Period genes (mPer1-mPer3) and two Cryptochrome genes (mCry1 and mCry2). The mPER:mCRY proteins feed back to inhibit CLOCK:BMAL1-mediated transcription by a mechanism that does not alter CLOCK:BMAL1 binding to DNA. Recent studies have shown that the transcriptional regulation of the core clock mechanism in mouse liver is accompanied by rhythms in histone H3 acetylation, and that H3 acetylation is a potential target of the inhibitory action of the mCRY proteins. The specific aims of this proposal will expand this discovery by defining the mechanistic details of the full repertoire of chromatin remodeling events that underlie the core transcriptional machinery of the clockwork, using molecular and biochemical approaches. The specific aims will 1) elucidate the temporal sequence of histone modifications that underlie rhythmic transcription in the clockwork; 2) use a proteomic approach to identify additional components of the circadian clock protein complexes; 3) delineate transcriptional mechanisms through nucleosome assembly; and 4) define the mechanisms by which the mCRY proteins inhibit CLOCK:BMAL1-mediated transcription. A long-term goal of this research plan is to define the cellular and molecular mechanisms that underlie circadian rhythms. The proposed studies provide an integrated approach that will define the key mechanisms of transcriptional control in the mammalian clockwork. Understanding the molecular clock could increase our knowledge of how clock gene mutations contribute to psychopathology (e.g., major depression and seasonal affective disorder). Likewise, such understanding should lead to new strategies for pharmacological manipulation of the human clock to improve the treatment of jet lag and shift-work ailments, and of clock-related sleep and psychiatric disorders